A method for controlling a compressor of an air-conditioning system is known, and in this method the power output of the internal combustion engine is increased, for example, by raising the engine speed or by increasing the quantity of air fed to the internal combustion engine, before putting the compressor into operation with a predetermined delay. In this method, the power output is increased each time by the amount of a predetermined fixed value. The delay before the compressor is switched on is necessary because the power output of the internal combustion engine cannot always be increased abruptly. It is also known, in a method of this type, to use a control unit for controlling the functions of an internal combustion engine. That means, the compressor of the air-conditioning system is no longer controlled directly by a thermostat mounted, for example, in the vehicle interior. Instead, the thermostat signal is sent to the control apparatus which, in response to this signal, first increases the power output of the internal combustion engine and then switches in the compressor with a delay. The increase in the power output of the internal combustion engine serves for keeping the speed of the internal combustion engine as constant as possible even when the compressor is switched on.
A disadvantage of this method is that the load on the internal combustion engine during switching-on of the compressor is not constant. For example, the switching-on of the compressor exerts a greater load on the internal combustion engine at high outside temperatures than at low temperatures. The load on the internal combustion engine when the compressor is switched on is higher when a high pressure prevails in the high-pressure region of the latter. This occurs when the compressor is switched on frequently because of high outside temperatures.
Therefore, in the known process and under specific preconditions, there are still pronounced fluctuations in power of the internal combustion engine when the compressor is switched on.